(a) Field of the Invention
The present invention relates to methods for preparing active carbons which can be used for water treatments, gas treatments. etc. More specifically, the present invention relates to methods for preparing active carbons having high strengths and high adsorption activities with a good productivity.
(b) Description of the Related Art
Known methods for preparing an active carbon by using a coal as a starting material comprise a step of pulverizing the starting coal to pulverized particles, a step of granulating the resulting pulverized particles, a step of carbonizing the resulting granules, and a step of activating the resulting carbonized granules.
In some of the known methods for preparing granular active carbons, as described in Japanese patent Application Publication No.48-412l0, etc., caking coals are used as the starting coals in order to obtain the active carbons having such high strength as required in uses of the active carbons. However, such known methods have disadvantages in that :
it is required to employ a fusion-prevention treatment for eliminating the tackiness of the surface of the granules obtained in the granulating step by oxidizing their surface in order to prevent the granules having a caking property from sticking with one another; it is necessary to increase temperature slowly in the carbonizing step in order to prevent the granules from foaming in the carbonization step: and a long activation time is required to obtain enough surface area required of active carbons.
In addition, those methods have some other disadvantages as follow. Since the particle size of the particles obtained by pulverizing the starting coal is as large as 100 mesh (0.140 mm), the numbers of the contact points between the sub-particles in the granules granulated therefrom as well as in the carbonized granules are small, causing the necessity for the use of a bituminous material such as a pitch. etc. as a binder for attaining an effective granulation. Further, the use of such a bituminous material as the binder is unfavorable to the activating reactions because the surface pores that the starting coal originally has are blocked by the bituminous matters having high carbon residue contents in the carbonization step, and it is difficult to prepare granules having particle sizes of not more than 1 mm because the particle sizes of the sub-particles that are to be granulated are large.